Method for providing improved lossy dielectric structure for dissipating electrical microwave energy

ABSTRACT

A LOSSY DIELECTRIC FOR DISSIPATING HIGH POWER WAVE ENERGY ON THE ORDER OF 100 TO 1800 WATTS COMPRISED OF A MAGNESIA MATRIX HOT PRESS-FORMED OF A DRY BLENDED MIX OF CALCINED MAGNESIUM OXIDE AND SILICON CARBIDE.

United States Patent US. Cl. 264-61 4 Claims ABSTRACT OF THE DISCLOSURE A lossydielectric for dissipating high power wave energy on the order of 100 to 1800 watts comprised of a magnesia matrix hot press-formed of a dry blended mix of calcined magnesium oxide and silicon carbide.

The invention herein described was made in the course of or under a contract or subcontract thereunder with the Navy Department.

This is a continuation-in-part of application Serial No. 586,649, filed Oct. 14, 1966 (now Pat. No. 3,538,205) and cross-reference is made to our divided application Ser. No. 104.810.

This invention relates to improvements in the method of increasing the efficiency of lossy dielectric materials in dissipating high power electrical microwave energy and the products obtained thereby. More particularly, the improvements concern the critical methods of providing an improved lossy dielectric material having the surprising ability to dissipate continuous power wattage on the order of 100 to 1800 watts without requiring external water cooling and can be operated under conditions up to 1200" C. without undergoing physical damage, in comparison to any known commercially available material not having this ability.

In the technical field of providing high performance lossy dielectric material for microwave devices and components, the prior art has devised porous ceramic structures with carbonized sugar, metal oxides and carbides, oxidized nickeliron alloys in sheet or wire form, and metallic surfaces coated with graphite. However, such structures have characteristics which reduce their use in airborne structures and particularly for use in dissipating high power microwave loads and as attenuators and terminations in traveling wave tubes.

It is accordingly an object of this disclosure to provide the art with a method of fabricating improved lossy dielectric material which improves the efficiency of high power electrical energy dissipation and high thermal conductivity under conditions of continuous wave power.

Another object of this improvement in the art is to provide an improved method of processing for obtaining improved lossy dielectric compositions having high thermal conductivity for efficient absorption and dissipation of heat energy generated under conditions of continuous high energy wave power and environment and induced high temperatures.

A further object of this improvement in the art is to provide a critical method of preforming lossy dielectric compositions for obtaining maximum thermal conductivity, minimizing porosity and attaining high uniform density to more efiiciently absorb and dissipate high power microwave electrical energy.

Further, additional objects and advantages will be recognized from the following description wherein the examples are given for purposes of illustrating the improved methods and compositions embodied herein. To

In general, the lossy dielectric composition of this disclosure consists essentially of critically prepared dielectric matrix and silicon carbide mixtures to provide a lightweight lossy dielectric of uniformly reproducible strength and properties. The disclosure consists essentially of the method for providing a matrix of selective materials whereby the dielectric properties are adjusted within certain limits to obtain improved operating characteristics for microwave attenuators or high power loads. The dielectric constant of the resulting material is within the range 9 to 20 and the loss tangents within the range 0.005 to 0.600, thus providing a range of property values capable of accommodating a wide spectrum of high power microwave energy absorption. Microwave attenuators absorb only a portion of the energy to prevent unwanted oscillations, usually at selected frequencies. Microwave loads (or terminations) are broadband devices designed to absorb and dissipate as heat as much energy as possible. The essential, unusual feature of our materials is that they combine an outstanding range of electrical properties with high density and high thermal conductivity, as a unique combination, not heretofore known to be obtained in the art.

The proportions of matrix and conducting granules vary to provide the required dielectric properties. Attenuators require from approximately 1% to 35% by weight of conducting granules. Loads generally require conducting granules in greater amounts up to about Thus, in the matrix herein provided the silicon carbide granules may vary from 1 to 80%. It is most important that the conducting granules be uniformly dispersed during the mixing operation so that all particles are separated and surrounded by dielectric matrix. Otherwise, the composite will act as a metallic conductor and will reflect rather than absorb energy. The initial preparation of preforming or molding and sintering operations in the preferred compositions are critical from the standpoint of attaining the highest uniform density to maximize thermal conductivity, strength and hardness, and to minimize porosity.

The following detailed descriptions are illustrative of processes and compositions used to prefabricate the herein provided improved lossy dielectric matrix expressed in grams (parts) matrix in which granules of an electrically conducting metallic carbide are essentially uniformly distributed and encapsulated.

First illustrated is a preferred magnesia-based ceramic material fabricated by hot pressing. It has highly superior energy absorbing and dissipating characteristics, making it capable of absorbing and dissipating 1800 watts of continuous wave power without failure and has exceeded the power handling capability of other material known to the art, making possible development and improvement in radar transmission.

EXAMPLE 1 (1) Grind the following for 2 hours in a 1 gal. porcelain mill with 6000 grams of alumina grinding media:

Magnesium oxide 800 grams Lithium fluoride22.8 grams Distilled water-1100 cc.

1 About 97 based on added dry Weight.

(2) Remove from mill and dry for at least 20 hours at 250 F. in an air convection oven.

(3) Micropulverize the dried material. (4) Mix dry the following for 10 minutes in a paddletype blender:

Grams Micropulverized powder of Step 3 350 Silicon carbide, 280 mesh 233 (5) Slowly add distilled water until a very thick but well blended paste is formed and continue blending for 30 minutes.

(6) Dry paste completely.

(7) Pulverize to pass a 14 mesh screen.

(8) Calcine in 600 gram batches to 1000 C. for 1 hour.

(9) Hot press 425 grams of the calcined material in a 2-inch diameter induction heated graphite die for ten minutes at 1550 C. under three tons total pressure. Allow to cool slowly and remove.

The composition of Example 1 may be modified within the following limits:

Table 1.-Lossy ceramic compositions Percent Magnesium oxide 20 to 99 Lithium fluoride to Calcium pyrophosphate 0 to 20 Manganese dioxide 0 to 10 Boron phosphate 0 to 20 Silicon carbide 1 to 80 The ceramic material of Example 1 is suitable for use in microwave loads and as terminations in traveling wave tubes. With an eight-gram specimen, 1800 watts of continuous wave power were dissipated without failure of the material. The best commercial material available, when fabricated to this geometry, failed catastrophically when subjected to about watts of continuous wave power. Similarly, the etficiencies of traveling wave tubes have been greatly increased by use of the composition described. The material is otherwise especially suited to applications where medium to low dielectric constants, high thermal conductivities and very high power dissipation are required.

Example 2, a less preferred modified self-glazing lossy matrix composition that is processed by dry pressing and sintering in a ceramic kiln, hence is more inexpensively produced and more amendable to high production, is provided as follows:

EXAMPLE 2 Pregrind the following ingredients for 16 hours at 55 rpm. in a clean, one-gallon porcelain ball mill jar containing 5500 grams of alumina grinding media:

Grams Alumina, 900 mesh 1 755.2 Manganese dioxide powder 18.4 Titanium dioxide powder 18.4 Bentonite 8.0 Water soluble organic binder (powder) 8.0 Distilled water (suflicient to form a paste) 950.0

1 About 94 based on dry weight of added ingredients. Final grind.Add the following and grind an additional two hours:

Grams Silicon carbide grain, 280 mesh 800 Microcrystalline wax emulsion, 46 /2% solids concentration (commercial grade) 112 Drying.Drain slip from mill into a glass or porcelain enamelled steel container and then dry at 100-l20 C, in an air convection oven for 2024 hours.

Granulation.Pare oif surface crust from the dried cake. Crush the remaining cake until it passes through an 8 mesh screen. Then alternately crush and screen until the dried material is sized between 28 mesh and 80 mesh for pressing.

Pressing.Press bars in a 2" x 1 steel die using 65 grams of the sized grain at 30 tons total pressure.

Organic burnout-Place pressed bars on a screen wire rack in an air convection oven. Heat to 140 C. in 4 hours at the rate of 30 C. per hour. Hold temperature at 140 C. for 20-24 hours, then increase temperature to 540 C. at the rate of not more than 100 C. per hour. Remove from oven after cooling below 140 C.

Firing-Place preburned bars on zirconia coated-silicon carbide kiln setters. Fire to 1400 C. for about one hour or until the pyrometric cone (15) is down.

The water soluble organic binder is added to the composition of Example 2, in powder form and is a synthetic or natural organic material, as gum arabic, gum tragacanth, polyvinyl alcohol, methyl, ethyl and the like cellulose and water soluble salts thereof, dextrin, sugar, polyethylene glycol, and the like, and mixtures of such binder materials as are commercially available and known to the art.

The composition of Example 2 may be modified as follows:

Table 2.Lossy alumina ceramic composition- Percent Silicon carbide, 100 to 300 mesh 1 to Alumina, 200 to 900' mesh 15 to 99 Manganese dioxide powder 0 to 6 Titanium dioxide powder Oto 6 Feldspar 0 to 20 Calcium pyrophosphate 0 to 20 Nepheline syenite 0 to 15 Bentonite .25 to 1 The material of Example 2 has been successfully used as a microwave load in an iso-duplexer to dissipate microwave reflections from an airborne radar antenna and as a high power broadband load material in the region of 100 watts or more average power dissipation, providing improved microwave switches and other electrical equipment. The material provides the appropriate dielectric constant and loss tangent for this application. The above fabrication process is more suitable for large production at lower cost than that of Example 1. This material, so prefebricated, possesses a high capacity for heat dissipation and high mechanical strength in proportion to its weight for airborne and other electrical components. Although this formulation does not have quite the power dissipating capability of the material described in Example 1, it dissipates at least an order of magnitude of more power without failure than the best commercial material available when fabricated in essentially the same geometry. Due to the improvement in thermal conductivity and ability to withstand high power loads of 100 watts or more average power dissipation, the composition of Example 2 provides an improved high power broadband load material for thermostatic switches and other electrical applications.

The combination of advantages of the material construction are indicated as follows:

(1) The material is unusually efficient in converting microwave energy to thermal energy. Size and weight of components can be greatly reduced.

(2) The material is very refractory and will therefore perform satisfactorily at high temperatures generated at high power levels during continuous operation. The material can be operated to 1200 C. without undergoing physical damage.

(3) The material inherently has a high thermal conductivity for efficient dissipation of the heat generated.

(4) The material is easily outgassed and does not interfere with typical bakeout cycles used in processing microwave power amplifier tubes. The outgassing products (principally sorbed gases) are readily removed.

(5) The material, being dense and hard, is readily machined to precision dimensions required for repeatability in performance.

(6) The formulation process described herein assures excellent reproducibility in preparing subsequent batches of the material.

(7) The material can be metallized and brazed into a metal enclosure which improves heat dissipation and resistance to shock and vibration.

(8) The material is very strong and rugged.

(9) The material has a thermal expansion that is compatible with other materials of construction in microwave devices.

(10) The dielectric properties of the material may be adjusted by composition changes so that matching the material to a particular application is possible.

Modification of preparing the magnesia matrix can be made by altering the steps as follows:

(1) Grind for about 2 to 16 hours a lossy ceramic forming composition consisting of the inorganic materials, magnesium oxide powder 20 to 97%, lithium fluoride to calcium pyrophosphate 0 to 20%, manganese dioxide 0 to boron phosphate 0 to 20%, with distilled water therewith suflicient to form a paste of said composition combination;

(2) Dry and calcine the mixture of step 1 to about 1000 C. for about one hour;

(3) Micropulverize the calcined mixture;

(4) Dry blend the said mixture with about 1 to 80% by weight particulates of silicon carbide of 100 to about 300 mesh; and

(5) Hot press the dry blended mixture of step 4 under high pressure and heating the molded form for a period of about 10 minutes to one hour in a temperature range up to about 1400 C. to 1550 C.

Having described the present embodiments of our improvement in the art in accordance with the patent statutes, it will be apparent that some modifications and variations may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of examples illustrative of our discovery, invention or improvement which is to be limited only by the terms of the appended claims.

What is claimed is:

1. The method of producing a strong lossy dielectric ceramic for microwave attenuators and high power microwave loads, said ceramic having a dielectric constant in the range of 9 to 20, with the loss tangents in a range of 0.005 to 0.600 and a high thermal conductivity capable of withstanding electrical induced temperature up to 120 C. and an eight-gram specimen of which is capable of dissipating 100 to 1800 Watts of applied wave power, the steps consisting of:

(1) Grind for about 2 to 16 hours a lossy ceramic forming composition consisting essentially of the combination of magnesium oxide powder 20 to 97%, and one or more of the group lithium fluoride 0 to 5%, calcium pyrophosphate 0 to 20%, manganese dioxide 0 to 10%, boron phosphate 0 to 20%, with a grinding media in a ratio of about 7 to 8 to about 1 part ceramic media, in combination with distilled water therewith suflicient to form a paste of said composition combination;

(2) Dry and calcine the mixture of step 1 to about 1000 C. for about one hour;

(3) Micropulverize the calcined mixture;

(4) Blend the said calcined mixture with about 1 to about 80% by weight particulates of silicon carbide of 100 to about 300 mesh;

(5) Hot press molding the dry blended mixture of step 4 under high pressure by heating the molded form for a period of about 10 minutes to one hour 6 in a temperature range up to about 1400 C. to 1550 C., and

(6) removing the said molded mixture from the mold.

2. The method of claim 1 wherein the said grinding is for a period of about two hours of the said magnesium oxide, present in an amount of about 97% dry weight, and lithium fluoride mix in said water, removing the mix from the grinding step and calcining the mix, micropulverizing the calcined material, mixing silicon carbide with the pulverized material, hot pressing the blended material in a heated mold in a temperature range of about 1400 C. to 1550 C.

3. The method of preparing a strong lossy dielectric ceramic capable of dissipating high energy microwave power comprising the steps of:

(1) mixing and mill grinding with a mill grinding media for a period of about 2 hours a lossy ceramic forming mixture consisting of magnesium oxide 20 to about 97 percent, in combination with one or more of lithium fluoride 0 to 5 percent, calcium pyrophosphate 0 to 20 percent, manganese dioxide 0 to 10 percent, boron phosphate 0 to 20 percent with the mill grinding media in a ratio of about 7-8 parts to 1 part ceramic forming mixture;

(2) remove from the mill and dry for a period comparable to 20 hours at 250 F.;

(3) micropulversize the dried material;

(4) mix and blend the micropulverized material of step 3 in the ratio of about 350 parts to about 230 parts powered silicon carbide on the order of 280 mesh with distilled water until a well blended paste is formed;

(5) dry the paste and pulverize the mixture to pass a 14 mesh screen;

(6) calcine the pulverized mixture on the order of 1000 C. for a period of on the order of 1 hour;

(7) hot press the calcined mixture in a die heated on the order of 1550 C. for a period on the order of ten minutes; and

(8) gradually cool the hot pressed mixture and remove from the die.

4. The method of claim 1 wherein said lossy ceramic composition consists of a mixture of about 800 parts magnesium oxide and about 23 parts lithium fluoride.

References Cited UNITED STATES PATENTS 2,716,190 8/1955 Baker 25263.5 3,143,413 8/1964 Kraph 264-332 3,365,631 1/1968 Delaney et a1. 252-63.2 3,376,367 4/1968 Subramanya et a1. 264-57 3,489,532 1/1970 Masuyama et a1 264-61 3,509,072 4/ 1970 Barrington et al. 252-516 FOREIGN PATENTS 1,067,918 10/ 1959 Germany 252-516 501,828 3/1946 Canada 264-61 873,825 7/ 1961 Great Britain 264-61 OTHER REFERENCES E. Carnall, Jr., Densification of MgO' in the Presence of a Liqiud Phase, Material Research Bulletin, 2[12], 1967, 1075-86.

JULIUS FROME, Primary Examiner I. H. MILLER, Assistant Examiner US. Cl. X.R. 

